Breeders face difficulties in two areas-breeding (including recording) and marketing. With regard to breeding, there are problems associated with correlated responses to selection. In most cattle breeding programmers, an aggregate index is constructed that includes traits such as growth, milk yield, fertility conformation, number of somatic cells in the milk, calving ease and duration of productive lifespan (for more details see Table 99).In dairy cattle, the main focus has been (and is still) put on milk yield, despite the negative genetic correlations between milk yield and reproduction and health-related traits. Undesired side-effects have, therefore, been observed-including lower fertility, and greater susceptibility to mastitis, leg problems and ketosis.